1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In recent years, low cost and space saving have progressed for image forming apparatuses such as copier or printers using an electrophotographic system.
Thus, miniature image forming apparatuses have widespread use not only in offices but also in small offices and individuals to be used for small amounts and various kinds of printing such as fliers, advertisement, and catalogs. Meanwhile, for the image forming apparatus, there has been a high demand for not only high image quality but also countermeasures of a wide variety of sheets.
Of the wide variety of sheets, particularly highly demanded sheets are coated sheets of which flatness and appearance are improved by using high-quality sheets as bases and applying paints to the surfaces. For the coated sheets, there is gloss, smoothness is high, photos or letters can vividly be reproduced, and finish quality is high. Therefore, the coated sheets are suitable for fliers, advertisement, and catalogs.
However, when the coated sheets are left in a bundle form in an environment of high humidity, the outer-layer surfaces absorb moisture and the sheets are mutually adsorbed with ease. When the sheets are mutually adsorbed, a problem may easily occur such as double-feeding in which overlapping sheets are conveyed from a sheet feeding portion or sheet feed failure in which a sheet is not conveyed. Thus, for example, Japanese Patent Application Laid-open No. 11-157686 suggests a technology for handling sheets by blowing air to side and upper surfaces of the sheets stacked in a sheet stacking portion so that adsorption between sheets is suppressed.
However, since sheet smoothness of the coated sheets is high, the coated sheets tend to be mutually adsorbed due to an electrostatic force of the mutual overlapping coated sheets. In particular, in image forming apparatuses of an electrophotographic system, a high transfer voltage is applied to a sheet when a toner image is transferred to the sheet. However, when a high voltage is applied to a sheet, a transfer current flows in the sheet, and thus the sheet is charged.
Here, in image forming apparatuses of the related art, the rear end of a preceding sheet to which a toner image is transferred in a transfer portion comes into contact with the upper surface of a subsequent sheet stacked in a sheet stacking portion depending on the sizes of the sheets or the sizes of the image forming apparatuses in some cases. Further, the sheet stacking portion is grounded to the earth in some cases. In this case, when a resistance value of the sheet stacking portion is small or a resistance value of all of the stacked sheets becomes small with a decrease in a stacking amount, a potential difference between a charged preceding sheet and a subsequent sheet increases due to the fact that the sheet stacking portion is grounded to the earth. In the case of coated sheets, this state occurs considerably.
As a result, an electrostatic force occurs between the preceding sheet and the subsequent sheet, the sheets are mutually adsorbed, and thus double-feeding of the sheets occurs. Since the double-feeding normally occurs unless the subsequent sheet is separated by a sufficient distance during a transfer operation of the preceding sheet, the double-feeding may not be prevented from occurring by the above-described technology for handling sheets by blowing air.
By improving an insulation capability of a sheet stacking portion, the double-feeding by the electrostatic adsorption can be prevented. However, when the insulation capability of the sheet stacking portion is improved, charge is gradually accumulated in sheets stacked in the sheet stacking portion and members constituting the sheet stacking portion during continuous feeding of the sheets. When the accumulated charge exceeds a given threshold value, a problem occurs in some cases, for example, in that an electric component or the like in an image forming apparatus erroneously operates due to electrostatic noise.
Thus, in the image forming apparatuses of the related art, when a preceding sheet comes into contact with a subsequent sheet in a sheet stacking portion during a transfer operation and the sheet stacking portion is grounded to the earth due to the downsizing, the double-feeding occurs in some cases due to the electrostatic adsorption caused by a potential difference between the preceding sheet and the subsequent sheet. Further, when the sheet stacking portion is not earthed to the ground, charge is accumulated in the sheet stacking portion and the constituent elements. When the charge exceeds a threshold value, a problem occurs in some cases, for example, in that an electric component or the like in an image forming apparatus operates erroneously due to electrostatic noise.